Many different processes for forming color images by transferring diffusible dyes are known. For example, in U.S. Pat. No. 3,134,764 a process using a dye developer in which a developing agent of hydroquinone type and a dye are bonded is described. Further, a process in which a diffusible dye is released upon an intramolecular transesterification is described in Japanese Patent Application (OPI) No. 63618/76 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), and a process in which a diffusible dye is released upon an intramolecular ring opening and closing reaction of an isooxazolone ring is described in Japanese Patent Application (OPI) No. 111628/74. In these processes, dyes diffuse to an image receiving layer in areas where development is not caused (unexposed areas) and the releasing and diffusion of dyes does not occur in areas where development is caused (exposed areas). However, these processes are disadvantageous because it is difficult to obtain an image having a high S/N ratio since the development and the releasing and diffusion of dyes occur simultaneously. In order to eliminate such a drawback, a process was developed in which a coloring material is previously converted to an oxidized form which does not have a dye releasing ability, the oxidized compound is developed together with a precursor of reducing agent coexistent and the oxidized compound is reduced by the reducing agent which remains without suffering the oxidation to release a diffusible dye. This process is described in Japanese Patent Application (OPI) No. 110827/78. However, this process has a disadvantage in that an image having a high S/N ratio can be obtained only when the development rate and the reduction rate of the coloring material by the reducing agent are strictly adjusted.
In accordance with another known process, a diffusible dye is released in areas where development is caused. In yet another process a diffusible dye is released by a reaction between a coupler having a diffusible dye in a coupling-off group and an oxidation product of a color developing agent. This process is described in British Pat. No. 1,330,524. In another process a diffusible dye is formed by a reaction between a coupler having a diffusion resistant group in a coupling-off group and an oxidation product of a color developing agent. This process is described in U.S. Pat. No. 3,227,550. In these processes using the color developing agent, however, image stains are formed due to oxidative decomposition products of the color developing agent which is a very serious problem.
Furthermore, as reducing dye releasing compounds which are construed as having the same function as the compounds according to the present invention, the compounds represented by the formulae (A) to (L) described below are known. ##STR2## As described in U.S. Pat. Nos. 3,928,312 and 3,993,638. ##STR3## As described in U.S. Pat. No. 4,053,312. ##STR4## As described in U.S. Pat. No. 4,055,428. ##STR5## As described in U.S. Pat. No. 4,198,235. ##STR6## As described in U.S. Pat. No. 4,149,892. ##STR7## As described in Japanese Patent Application (OPI) No. 104343/76. ##STR8## As described in Japanese Patent Application (OPI) No. 104343/76. ##STR9## As described in Japanese Patent Application (OPI) No. 104343/76. ##STR10## As described in Research Disclosure, No. 17465 (1978). ##STR11## As described in U.S. Pat. No. 3,725,062. ##STR12## As described in U.S. Pat. No. 3,728,113. ##STR13## As described in U.S. Pat. No. 3,443,939.
The compounds represented by the formulae (A) to (I) above are oxidized upon development processing and then release by the action of an alkaline diffusible dye having a sulfamoyl group at an end of the molecule thereof. However, these processes have several drawbacks in that they require an alkaline solution in a high concentration as a developing solution and in that since the dye released has a sulfamoyl group and its diffusibility in a hydrophobic binder is extremely low, it is necessary to use a hydrophilic binder and in addition to use an alkaline solution as a diffusion accelerator in order to obtain images having a high density. Also, the compounds represented by the formulae (J) to (L) have a low efficiency in the dye releasing step after they are oxidized and thus they cannot be applied to practical use.